Power Steering

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Why Replace Your Power Steering Fluid?

The power steering system is rarely considered for regular fluid change but it is no different from any other subsystem in the car with fluid as the working media.  Consisting of a pump, steering box, and reservoir, the system is actually a sophisticated hydraulic servo actuator.  Fluid circulates rapidly around the circuit starting and ending in the reservoir.  It is rumored that there is a permanent filter in the reservoir. Pressure in the system is developed only when steering assist is needed, sensed by small errors in position between the steering wheel input shaft and gearset pinion.
The fluid used in the system varies among several but is marked on the filler cap:  ATF, CHF 7.1 or CHF 11S, of which I know the latter is a synthetic type fluid at approx. $25 per liter.  You must use the indicated fluid and cannot mix them.

ps_resevoir.jpg (35164 bytes)  Fluid type is marked on the cap of the power steering reservoir

 

How To Do The Job

Allow 1 hour for this task.  It is not practical to flush the entire system without special equipment but you can dilute the old fluid by about 60%.  Doing this once every 2 years will keep the fluid in very good condition.  Remove the front splash guard (under the engine fan) and locate the drain plug on the pump body.  Remove the 8mm hex socketed plug and drain whatever fluid will come out.  Replace the plug and splashguard, refill the reservoir with new fluid, start the engine and exercise the steering.  Continue to add fluid up to the "max" mark as air is purged over a few minutes.  The pump will make a lot of (relatively harmless) noise while air is in the system.

A modification you can make...

It's well known that the E34's power steering hoses typically fail around 100k miles.   Complaints range from leakage at clamps to actual failure of the hose rubber resulting in seepage through the hose.  Specifically on the 530/540, I believe that the hoses are situated too close to the left exhaust manifold and will be subject to excessive radiated heat.  You will note that some of the hose sections are already heat shielded - why did they leave other sections unprotected?
ps_hose_mods.jpg (45912 bytes)pwr_str_mod.jpg (67943 bytes)
Take some kitchen-variety aluminum foil and wrap it around the hose sections exposed to the left manifold.  You don't have to do a great job to get much better protection.

A Recent Failure, ...and a Fix

One of my PS hoses suddenly started leaking at the connection to the reservoir - the one towards the rear. For some reason the crimped hose clamp was loose - either it had not been very tight from new or the hose had extruded out from under it. Here's how I fixed it.
Remove the two nuts holding the reservoir bracket to the wheel well. Lift away the reservoir and wrap a large rag around it to prevent scratching the plastic valve cover. Cut a piece of coat hanger wire about 1.5" long and fold it into a "U" shape with about 1/4" between the legs. Place it around the crimped part of the clamp in the little grooves. Get out your trusty vise grip pliers and clamp them over the wires, pressing them into the grooves. Adjust the screw on the pliers such that you can utilize the maximum squeezing force to further compress the clamp. Don't tighten too much - the reservoir nipple is plastic - just enough so that it won't turn.
A week later it was leaking again. I cut 1/2" off the top of the hose and installed a standard hose clamp. So far it's OK.

Other Problems

A number of owners have reported loud noises and engine stalling while driving at stationary or low speeds. It appears that the system is right on the edge of its performance envelope during these conditions. It's the nature of servo systems to be intolerant of an insufficient power supply, in this case fluid flow rate. Severe instability can result in the feedback loop of the PS, which seems to be what's happening. During this condition the load on the pump could be very high, resulting in an engine stall if the belt is up to it.
Although it's very possible that BMW have inadvertently undersized the pump pulley resulting in insufficient flow at idle on some 530i examples, it's just as likely that driving technique is to blame. I suspect that US drivers are more likely to crank hard on the wheel while the car is not moving as we are more used to overcooked PS in our cars. Europeans are more likely to get the car moving slightly first. In any case, simple turning the wheel more slowly and/or getting the revs up a bit should resolve the problem, assuming the PS system and belts are up to snuff.
It is not likely that the filter or fluid condition would affect this - those variations are way too subtle, but an old belt or defective idler could slip under high load and cause an additional loud noise.